Electronic equipment is susceptible to damage owing to high spike voltages being coupled to the A-C power mains from which they obtain operating energy. Such spike voltages can be caused by surges arising from inductive loads, lightning storms, or the like.
The highest potential transient spike voltages are short in duration, typically tens of microseconds. These high potential spikes can range into many thousands of volts, and many thousands of amperes. Such high voltages tend to overstress electronic components, subjecting them to hazardous and unwanted operating conditions. Owing to the low source impedance of the A-C mains and very high potentials involved, very large currents flow during component breakdown. As a result, dissipation in the components may be substantial, often resulting in their being destroyed.
Heretofore, it was customary to use power mains frequency isolation transformers in D-C power supplies obtaining energy from the A-C power mains, to isolate the power mains voltage from the equipment for safety reasons. The self-inductance and distributed capacitance of the isolation transformer windings integrated the energy contained in transients caused by lightning or other sources received from the A-C power mains. As a result, voltage excursions delivered to the load were minimal and were current limited by the transformer. More recently, to lower cost and reduce size and weight, there has been a tendency to eliminate mains isolation transformers. Instead, the A-C mains voltage frequency is rectified directly to supply direct operating voltage to a switched-mode voltage regulator. This regulator, in turn, provides electrical isolation via a high frequency transformer that supplies subsequent electronic equipment (such as a television receiver or computer equipment). This newer design (switched-mode supply) electronic equipment has proven to be undesirably susceptible to damage from surges or transients on the A-C power mains.
In response to this problem, a number of line surge suppressors have been marketed, though none to the knowledge of the present inventor has offered satisfactory protection for electronic equipment during lightning storms with reasonable life, cost and reliability. A number of available line surge suppressors contain voltage limiting devices (usually Metal Oxide Varistors--MOVs) for connection in shunt across the A-C power mains, which voltage limiting devices have very large ONE-TIME surge current handling capability--e.g. 4500 or 6500 amperes. Spark gaps, gas discharge tubes and zener diodes also have all been employed, but each such component has either a serious reliability problem or an operational characteristics problem, or both, as they are applied in power line surge suppressors (see, for example, U.S. Pat. Nos. 4,563,720--Clark, 3,793,535--Chowdhuri, 4,068,279--Byrnes, 4,463,406--Sirel, 4,628,394--Crosby et. al.).
A common problem with the generally accepted shunt protector design concept is that the A-C mains have a very low source impedance, so the shunt voltage regulation afforded by these voltage limiting devices is severely compromised due to the lack of current limiting. Furthermore, the high current rating of 4500 or 6500 amperes of such varistors is a ONE TIME rating, after which the characteristics of the metal oxide varistor are permanently compromised for subsequent transients. What has been found to be needed to make the shunt voltage regulation of the limiting devices effective is a series pass element to augment the impedance of the A-C power mains and the A-C line cord. Unless such a series-pass element is provided, a high energy transient applied directly across the varistor or other voltage limiting device is supplied from a very low source impedance that is capable of continuing to support current flow at unpredictably high currents, perhaps approaching or exceeding the limit of the surge current handling capability of the voltage limiting device, and compromising performance for subsequent transients. The thousand ampere or more surge current is accompanied by a several hundred volt peak voltage developed across the voltage limiting device, and the electronic equipment receiving operating voltage from across the voltage limiting device often succumbs to the overvoltage. This is so even though (as the manufacturers of the line surge suppressors point out) the line surge suppressor itself is capable of withstanding the rated lightning-caused surge at least once before becoming ineffectual. Furthermore, there is generally no indication given when the surge suppressor has experienced such an event and the transient protection of the varistor no longer meets the original specifications.
In order to provide the necessary protection, a series pass element is required between the A-C mains and the voltage limiter which is able to withstand 6,000 volts peak, which carries the line current and exhibits sufficiently low impedance that its insertion loss during normal operation at A-C power mains frequency is reasonably low, but exhibits high impedance and high loss during voltage spikes. While such requirements may be met by a suitably large inductor, in order to obtain sufficient impedance to limit the current flow in the voltage limiting device to a safe value during transient spikes in excess of 2000 volts on the A-C power mains while preventing the appearance of overvoltage or overcurrent at the voltage limiting device (typically a varistor--which may be required to dissipate over 1,000,000 watts during a transient), the inductor will be unduly expensive and bulky. Furthermore, problems arise with using inductors having high permeability magnetic structures (which is often done to reduce size and cost) in that high peak energy levels often associated with lightning-caused transients cause saturation in these magnetic structures and attendant loss of inductive reactance when it is needed most. That is, the series pass impedance is lower than required during such transients unless the magnetic structure is massive. Inductors with air gaps and low permeability magnetic structures, such as those of soft iron, may be used since they are less prone to characteristic shifts at high currents, and yet provide the required losses to protect equipment from transients.
It has been found to be difficult to obtain consistently adequate shunt regulation against high energy surge voltages utilizing only one stage of voltage limiting. Furthermore, voltage limiting devices can survive high energy surges indefinitely only if the current is properly limited. For these reasons, a cascade of shunt voltage limiters has been found to be desireable to protect a load such as a switching regulator from these surges on a repeated basis. The question then is what kind of shunt limiters should be used to provide safe voltages (less than 250 volts peak for 110 volt r.m.s. nominal A-C mains voltage, or about 1.5 times normal peak voltage) to the protected device, and still operate within reliable dissipation limits.
The prior art recognizes that voltage limiting devices tend to be slow acting, and that spikes caused by lightning and other high energy sources are of only tens of microseconds duration. Small value capacitors have been used in parallel with voltage limiting devices to shunt high frequency energy away from the equipment being protected by the line surge suppressor. Here too, previous designs tended to provide no (or an inadequate) series pass element, and the capacitors were of insufficient value to provide any degree of protection (being used mainly to provide high frequency noise filtering). Varistors have wide clamp tolerances, finite clamp impedance and limited life for high current transients. A typical varistor for nominal 110 volt A-C mains protection such as a GE Type V130LA10A is rated at 200 volts clamp onset voltage (a satisfactory value), with ability to withstand 4500 amperes surge current only ONE TIME, after which permanent changes take place that compromise performance. Moreover, the clamping level of that varistor is specified as 340 volts at 50 amperes of surge current (already an excessive voltage for 110 volt nominal mains voltage), with clamping voltage increasing in proportion to higher surge currents, up to about 500 volts at 2200 amperes, HALF the maximum rated current (a dangerously high voltage for many loads). Specification #587 of the Institute of Electrical and Electronics Engineers (IEEE) recommends that surge suppressors be suitable for use with a 6,000 volt, 3,000 ampere current source for major feeders, short branch feeders, and load centers.
Unless the current is limited to impractically low levels (30 GE Type V130LA10A), varistors would prove unreliable in the first stage. This is because any inductor that could limit a 6,000 volt transient to 30 amperes (approximately 1 millihenry) would have appreciable loss at the A-C mains frequency, and be physically large. The clamp voltage tolerance makes varistors impractical for the second stage, since the voltage transient should be limited as noted above, to 250 volts peak for 110 volt nominal A-C mains voltage for reliable protection. For example, a sample varistor with a nominal 200 volt onset clamp has a specification ONSET voltage that ranges from 184 volts to 228 volts, and therefore is likely to produce a much higher voltage under actual transient conditions.